1. Technical Field
The present invention relates to the field of magnetic tape drives. More particularly, the present invention relates to positioning a magnetic head for proper pickup of information tracks on a magnetic tape or disk.
2. Description of Related Art
In magnetic tape drive technology, it is known to employ a tracking servo system to follow pre-written servo tracks on the magnetic tape to accurately position the magnetic heads. Typically, one or more read heads are dedicated to sensing the servo tracks and are added to or interleaved with the data read heads. For example, a combination head with 18 side-by-side data heads may include two or three servo read heads to assure accurate positioning. As the width and spacing of the data tracks is decreased to increase capacity, it becomes more difficult to maintain the necessary positional accuracy using a narrower magnetic servo track. If the width of the servo track is left wider to avoid this problem, some of the potential capacity increase is lost, and the head becomes more complex due to the unequal spacing of the elements. For the magnetic tracking method to work with an unstructured single-element read head, complex encoding of the track and signal processing is required to extract a position error signal.
Previously, optical tracking systems for magnetic tape drives have been proposed which use servo tracks on either the front (magnetic) side or back side of the tape. M. L. Leonhardt and S. D. Wilson disclose in “Optical Servo System For A Tape Drive” U.S. patent application Ser. No. 08/980,723, filed on Dec. 1, 1997, now U.S. Pat. No. 6,084,740 the use of conventional optics with servo tracks on the back of the tape, as well as reference marks on the sides of the magnetic head. This system enables accurate relative positioning of the tape and head without the need for rigid positioning of the optical system relative to the magnetic head. However, the large-scale optics required are difficult to implement in a small form factor drive, typically having limited space adjacent to the magnetic head.
Archibald Smith discloses in “Integrated Optical Tracking System For Magnetic Media”, U.S. patent application Ser. No. 09/203,784, filed on Dec. 2, 1998, now U.S. Pat. No. 6,275,349 an optical system which uses miniature opto-electronic modules with servo tracks on the front side of the tape. The opto-electronic modules are integrated into the magnetic head to achieve a compact structure. Both of these systems illuminate a region of the servo tracks with LED light sources and focus the tracks onto a segmented detector that senses their position. Accurate focusing is required, which places stringent dimensional tolerances on the system.
S. W. Farnsworth and S. D. Wilson disclose in “Optical Servo System For Magnetic Disk,” U.S. Pat. No. 5,121,371, Filed on Jun. 18, 1990 and again in “Diffractive Optical System For Tracking On FlopticalR Disks,” SPIE, vol 1960, pp 72–79 (1992), other prior art systems related to optical tracking for floppy magnetic disk drives. Using this system, the need for accurate focusing onto segmented detectors is avoided. This system uses two groups of small light spots. There are typically four to six strong spots in each group, having the same separation as the servo tracks. The two groups are offset in the transverse direction by one quarter of the servo track spacing, and separated in the longitudinal direction. Each group of spots is a fringe pattern produced by the interference of light from a double slit mask or hologram illuminated by a laser. Each group of spots is imaged onto a separate non-segmented detector, whose output varies depending on the position of the fringes relative to the servo tracks. The focal depth for the optical sensors described above is +/−100 um compared to +/−1 um for the previously referenced systems.
Problems associated with the prior art include the continued need for a separate encoding head in order to accurately judge the position of the magnetic head relative to the tape guiding structure. Other problems include inaccurate positioning of the tape head due to dirty or damaged servo tracks on the magnetic tape. Still other problems with the prior art include focusing problems associated with the segmented detectors and slow response or positioning overshoot due to configurations which actuate more than just the mass of the magnetic head. In an effort to solve the above-mentioned problems, the present invention is disclosed.